This invention relates to the field of radio frequency identification transponders (RFID tags), and particular applications thereof involving vehicle identification and parking control systems.
RFID tags receive RF electromagnetic radiation from an interrogating transmitter and return information recorded in the tag to a receiver and computer which is usually associated with the interrogator transmitter.
RFID tags can and have been used in many ways for locating or identifying tagged objects including animals, people, vehicles, or other objects either stationary or mobile. Usually the RFID tag returns distinctive information from the particular tag which may be variable or invariable in nature.
As shown in U.S. Pat. No. 3,098,971 to R. M. Richardson, it has long been proposed that the power necessary to transmit the return signal from the transponder be provided by the much stronger signal that is received by the transponder. This eliminates the necessity for a battery or other power source for the tag. Such an RFID tag is referred to as a passive tag, and it is the preferred form for use in the present invention. The present invention is not limited to such passive tags, however, and could be implemented with battery powered tags.
In an RFID tag of the passive type an interrogator signal picked up by the antenna of the tag induces an alternating current in an antenna circuit which may be rectified by an RF diode, and this rectified current can be used in a power supply for the electronic components of a microcircuit. A digital memory element of the microcircuit stores identification information and/or other data. A lower frequency signal generated in the microcircuit is caused to modulate the return signal transmitted from the RFID antenna thereby communicating information coded in the lower frequency signal modulation may be implemented either by altering the antenna loading or by other suitable form of modulation. Thus the RFID tag may be interrogated by a signal which both communicates with the RFID tag and supplies the power for its circuit so that the RFID tag can respond with an information carrying signal from its transmit antenna, all without requiring a battery or other power source for the RFID tag. In some systems information in signals received from the interrogator may be stored in a digital memory of the transponder as well.
Further refinements in RFID tag technology were made as shown in U.S. Pat. No. 4,075,632 to Baldwin et al. wherein tags were proposed with logic and read/write memories and transistors for modulating the return signal were also powered by the energy received by the transponder. Such refinements are also shown in U.S. Pat. No. 4,786,907 to Akoelle.
While improvements in semiconductor technology to provide microcircuits which are smaller and have lower power requirements have increased the capability of RFID tags and the systems which employ them, there are limitations which have still not been entirely overcome, particularly in the passive type of RFID tags. Use of such tags where the distance between the interrogator and the transponder is more than a few feet or about one meter presents difficulties. The amount of power transmitted by the interrogator is subject to regulations as well as practical limitations so that the effect of the well known square-of-the-distance power reduction factor militates against reliable use at longer ranges. There is an associated problem exacerbated by the low magnitude of the return signal power in that frequently more than one RFID tag is in the area being interrogated, presenting the likelihood that interference between return signals from different tags will adversely affect the reliability of a system.